1. Technical Field
The present invention relates to a bending vibration piece having a vibration arm which performs bending vibration and a method for manufacturing the same. The invention also relates to various electronic devices using this bending vibration piece.
2. Related Art
According to related arts, a piezoelectric device such as oscillator or real-time clock module equipped with a piezoelectric vibrator, piezoelectric vibration piece and IC chip as a clock source for an electronic circuit is broadly used in electronic devices such as timepieces, consumer electronics, various information and communication devices and office automation appliances. Also, in various electronic devices such as digital still camera, video camera, navigation device, vehicle body attitude detection device, pointing device, game controller, mobile phone and head-mounted display, a sensor such as piezoelectric vibration gyro using a bending vibration piece is widely used to detect physical quantities such as angular velocity, angular acceleration, acceleration and forces.
As a bending vibration piece for a piezoelectric vibration gyro, a tuning fork-type bending vibration piece having a pair of vibration arms extending parallel to each other from a basal part is known (see, for example, JP-A-9-14973 and JP-A-2002-340559). Also a double-side tuning fork-type bending vibration piece having a pair of drive vibration arms extending parallel to each other on one side from a basal part and a pair of detection vibration arms extending parallel to each other on the other side is known (see, for example, JP-A-2002-340559 and JP-A-2007-93400).
Many bending vibration pieces are formed by photoetching a wafer of a piezoelectric single crystal material, for example, crystal or the like, to work the wafer into a desired outer shape and then patterning an electrode film on a surface of the wafer. Since the piezoelectric single crystal material such as crystal has etch anisotropy, the vibration arms have a bilaterally asymmetrical shape instead of having an ideal rectangular cross-section. Also, if there is misalignment of a photomask in the working of the outer shape of the bending vibration piece, the cross-section of the vibration arms may become vertically asymmetrical in the direction of thickness. In such vibration arms with an asymmetrical cross-section, a tensile force and a compressive force due to an electric field generated inside by an alternating current applied to drive electrodes act in an unbalanced way between the upper side and lower side of the cross-section. Therefore, the vibration arms perform bending vibration in a direction of width while being displaced in an out-of-plane direction, that is, a direction of thickness, instead of an in-plane direction parallel to main surfaces of the vibration arms.
In the tuning fork-type vibration piece, vibration of the vibration arms in the in-plane direction causes a vibration leakage on the basal part side and leads to a rise in CI value or variation in CI value among vibration pieces. Also, in the bending vibration piece for a piezoelectric vibration gyro, when the drive vibration arms vibrate in the out-of-plane direction in driving mode, the detection vibration arms vibrate unnecessarily and a detection signal is outputted even when the vibration piece does not rotate. Therefore, detection sensitivity and accuracy are lowered.
To solve such a vibration leakage, bending vibration pieces with various measures are developed. For example, an angular velocity sensor which includes a vibrator made up of a piezoelectric body formed in the shape of a tuning fork including a pair of arm portions and a connecting portion connecting the arm portions, in which a ridgeline near the base of the arm portions is cut by machining so as to take weight balance between the two arm portions, is known (see, for example, JP-A-11-351874). Moreover, a technique of achieving a surface roughness of 2 μm or less in the ground part of the ridgeline of the arm portions in this angular velocity sensor and thus taking precise weight balance between the two arm portions and improving output characteristics is known (see, for example, JP-A-2002-243451).
When the vibration arms of the bending vibration piece are mechanically cut in this way, work dust generated in the cutting may re-attach to the vibration arms, deteriorate vibration characteristics and thereby lower detection accuracy. Thus, a method for manufacturing a vibrating body, including forming an electrode on a vibrating body that is worked in an outer shape thereof, forming a protection film thereon, then working and removing a portion of the vibrating body, and then removing the protection film to remove re-attached dust, is known (see, for example, JP-A-2006-214779).
Moreover, mechanical cutting may damage the vibration piece and it is difficult to control the amount of working accurately to remove a vibration leakage. Thus, a technique of depositing silicon dioxide, metallic material or the like by vapor deposition or the like on a corner part of the base part of the vibration arm situated on the side to which the direction of vibration of the vibration piece is inclined when the vibration piece vibrates in the out-of-plane direction in driving mode, thus increasing the rigidity of this part to make this part harder to deform, modifying the direction of vibration and removing a vibration leakage, is known (see, for example, JP-A-10-170274).
However, in the above related-art technique of removing a vibration leakage, the work of adjusting the amount of a substance attached to the corner part of the base part of the vibration arm in accordance with the magnitude of leak vibration and controlling the rigidity of this part to remove the leak vibration is very difficult in practice. Particularly when the vibration piece is downsized, it is more difficult to control the attachment of a substance.